1. Field of the Invention
The present invention relates to a thermosetting resin composition. More specifically, the present invention relates to a thermosetting resin composition which, by means of the addition of a cured powder having a good affinity for the thermosetting resin, affords a cured product which has a reduced water infiltration compared to prior cured powders, as well as an excellent flexibility, a low coefficient of thermal expansion, and a low mold shrinkage ratio.
2. Description of the Prior Art
Thermosetting resin compositions have excellent electrical properties, including dielectric properties, volume resistivity, and dielectric breakdown strength, in addition to excellent mechanical properties, such as flexural strength, compression strength, and impact strength. As a consequence these compositions are widely used as insulating materials in various electric and electronic parts through the use of such methods as transfer molding, injection molding, potting, casting, powder coating, immersion, and dripping.
A disadvantage of thermosetting resins is that they are generally rigid. When these resins are used, for example, to seal electric or electronic parts, large mechanical stresses are imparted to the interior elements. As a consequence, the elements may not function properly or parts of the elements may be broken. The stress develops due to the difference in thermal expansion coefficient and post-molding shrinkage ratio between the thermosetting resin and the elements of electric or electronic parts. Electric and electronic parts have very low thermal expansion coefficients and shrinkage ratios, while the resins have large values for these characteristics. These large difference results in generation of the aforementioned excessive internal stresses to the elements and other constituent materials of electric/electronic parts during molding, baking and subsequent thermal cycles.
The difference in thermal expansion coefficients and post-mold shrinkage is also responsible for the appearance of cracks in the thermosetting resin itself and for the appearance of spaces between the electric/electronic component and the thermosetting resin. These participate in deterioration of the element due to the infiltration of water and other impurities into these spaces.
While their object has not been to improve the thermal expansion coefficient and post-molding shrinkage ratio of thermosetting resins, efforts have been made at improving the properties of thermosetting resins. For example, Japanese Patent Publication No. 77/36,534 concerns an improvement in the lubricating properties of the surface of resin moldings which is achieved by blending organopolysilsesquioxane powder into phenolic resins, and Laid Open Japanese Patent Application (Kokai) 52/14,643, published on Feb. 3, 1977 relates to increasing the abrasion resistance to metal through the use of a synthetic resin filler comprising the finely divided powder of a cured material based on organopolysiloxane and an inorganic filler. However, these are both unsatisfactory in terms of the thermal expansion coefficient, post-molding shrinkage, and flexural modulus.
U.S. Pat. No. 4,663,397, which issued to Morita and Shirahata on May 5, 1987 discloses thermosetting resin compositions comprising 100 parts by weight of a continuous phase of a curable thermosetting resin composition and from 1 to 100 parts by weight of a finely divided, cured polyorganosiloxane resin wherein at least 90 weight percent of said polyorganosiloxane resin consists essentially of R.sup.1 R.sup.2 R.sup.3 SiO.sub.0.5 units and R.sup.4 SiO.sub.1.5 units, with any remainder consisting essentially of at least one member selected from R.sup.5 R.sup.6 SiO.sub.0.5 units and SiO.sub.4/2 units, where each of R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 represents a hydrogen atom, a monovalent hydrocarbon radical, or a monovalent substituted hydrocarbon radical, and the molar ratio of R.sup.1 R.sup.2 R.sup.3 SiO units to R.sup.4 SiO.sub.0.5 is from 0.1 to 3.
In their copending U.S. application Ser. No. 29,037 filed on Mar. 23, 1987 the present applicants disclose curable thermosetting resin compositions comprising a continuous phase of a curable thermosetting organic resin or a curable thermosetting silicone resin containing a dispersed finely divided particles of a cured composition prepared by reacting (1) an organopolysiloxane, (2) a curing agent for the organopolysiloxane, (3) an alkoxy substituted organosilicon compound and (4) a compound of aluminum or zirconium where said compound is an alkoxide, phenoxide, carboxylate or a derivative of a beta-dicarbonyl compound or a o-hydroxyketone. The presence of ingredients (3) and (4) imparts improved adhesion between the continuous and dispersed phases of the composition.
In Japanese Laid Open Patent Application (Kokai) No. 58/219218, published on Dec. 20, 1983 the present inventors proposed another solution to the aforementioned problems of reducing internal stresses and the mold shrinkage ratio of thermosetting resin compositions by blending a finely divided cured material containing 10 wt % linear siloxane fraction into a thermosetting resin.
Furthermore, in Japanese Laid Open Application (Kokai) No. 59/96122, published on June 2, 1984, the present inventors propose a method using a spherical cured material which is cured to the elastomer while in the sprayed state. However, when any of the aforementioned finely divided cured materials or microfine spherical cured material is blended into a thermosetting resin, the problem arises that water easily infiltrates into the gaps generated between the thermosetting resin and the finely particulate cured material as a consequence of the latter's poor affinity for the thermosetting resin. This is particularly true of material consisting of 100 wt % linear siloxane fraction.
Taking into consideration these problems residing in the prior art, the objective of the present invention is to provide, by means of the addition to the thermosetting resin of a finely divided cured material exhibiting a time-invariant affinity for the resin, a thermosetting resin composition which affords moldings having a reduced water infiltration compared with the use of prior finely divided cured materials, and in particular having a high flexibility, a small thermal expansion coefficient, and a small mold shrinkage ratio. Furthermore, this is to be accomplished without mold soiling or exudation onto the surface of the cured product during molding.